Solid state lasers

ABSTRACT

A solid state laser has an elongate slab of lasing material having a rectangular cross section with the lower face of the slab contacting a slab mount which is of a high thermal conductivity material. Energy to drive the lasing medium is provided by a flash lamp. Upper and lower faces of the slab are polished to an optically smooth finish so that light is able to propagate in a generally axial direction through the slab. Side faces of the slab are polished and then re-roughened to provide a finish with a surface damage zone comparable in depth to the wavelength of the lasing emission. For a lasing wavelength of one micrometer, the depth of surface damage is in the region of one micrometer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.09/000,199, filed Jan. 27, 1998 which is a 371 of PCT/GB96/02592 filedOct. 22, 1996, now U.S. Pat. No. 6,039,632.

The present invention relates to solid state lasers and in particular,though not necessarily, to solid state lasers employing a slab-typelasing medium.

Many conventional solid state lasers employ a cylindrical lasing rod,for example of Nd:YAG, with mirrors placed at opposed ends of the rodLasing light propagates axially backwards and forwards along the rodcausing amplified stimulated emission to occur. A problem with thisarrangement is that the optical pump light applied to the rod generatesheat which in turn gives rise to temperature gradients across the rod,i.e. transverse to the direction in which light propagates. Thetemperature gradients cause non-uniformities in the optical propertiesof the rod to arise, causing distortion and power loss in the lightoutput of the laser. Whilst it is possible to alleviate the steady stateproblem by various means including use of liquid coolants, the problemof dynamic changes in temperature remains significant with respect tolasing performance.

More recently, rod-type lasing media have been replaced with elongateslabs having a rectangular or square cross-section, in an attempt tofurther reduce the problems caused by temperature gradients within thelasing media. In a slab-type medium, light propagates lengthwise alongthe medium in a zig-zag manner, reflecting alternately off the twoopposed longer side faces of the slab which are polished smooth tomaximise internal reflections. This is illustrated in FIG. 1.

The zig-zagging of the light path effectively averages out the effect oftemperature gradients ΔT_(b) between the two opposed faces 1,2 fromwhich the light reflects, reducing distortion of the light beam andtherefore improving collimation of the laser output beam. To remove heatfrom the slab it is usually cooled via one or both of the internallyreflecting faces 1, 2, e.g. using a liquid coolant.

With slab-type lasing media however, there still remains the problem oftemperature gradients ΔT_(a) arising across the width of the media, i.e.between the side faces 3, 4 from which the light beam is not reflected.

It is an object of the present invention to overcome or at leastmitigate disadvantages of known solid state lasers.

It is a further object of the present invention to reduce heatgeneration within solid state laser media and to reduce temperaturegradients arising therein.

According to a first aspect of the present invention there is provided amethod of reducing temperature gradients within an elongate solid statelasing medium, the medium comprising one or more substantiallynon-reflecting faces for emitting or scattering radiation generated byamplified spontaneous emission (ASE), the method comprising treatingsaid non-reflecting face or faces to reduce the amount of heat generatedby radiation passing therethrough.

Where the medium is provided with one or more pre-roughened faces, theor each face may first be polished visually smooth and then reroughened,such that the depth of surface damage at the roughened face is less thanthat of the original face. Thus, the scatter path at the roughened faceis reduced and heat generation, due primarily to pump light, is alsoconsequently reduced. “Roughening” may be taken to include producing asurface finish which scatters incident light. The finish may compriseperiodic or random patterning.

Alternatively, faces of the lasing medium through which it is requiredto emit or scatter parasitic ASE light can be polished visually smoothand coated with a material whose thermal and optical properties are thesame or similar to those of the lasing medium, but in which heatdissipation is less than that in the lasing medium. The outer surface ofthe coating is then roughened to provide a substantially non-reflectingscattering finish. Given the relatively low heat dissipation whichoccurs within the coating material, even a relatively large scatter pathat the outer surface of the coating material will result in a relativelysmall amount of heat generation compared to that which would occur at aroughened surface of the lasing medium. Where the lasing medium is inthe form of a slab, the surface coating may take the form of thinsections of undoped lasing material or dielectric bonded to the polishedshort faces of the slab. Alternatively the coatings may be depositedusing thin film deposition techniques.

The reduced level of heat generation at the periphery of the lasingmedium reduces the need for heat sinking or thermal impedance matchingto the side faces. In the case of a slab-type medium, this makes itpossible to thermally isolate the slab on three side faces with a gasfilled or vacuous gap (thereby reducing the affect of externaltemperature variations) and to provide a heat sink on only one of thebeam reflecting faces to permit heat removal.

In contrast to conventional approaches to reducing the effects oftemperature gradients within a solid state lasing medium, whichgenerally involve increasing the conductivity of heat within or aroundthe lasing medium, the present invention relies upon reducing the levelsof heat generation within the lasing medium itself.

This reduction has been achieved as a result of realising thesignificant role which the rough surface finish and resulting depth ofsurface damage (i.e. crystal discontinuity) given to faces of lasingmedia play in the generation of heat within the media. In order to allowparasitic light generated by amplified spontaneous emission (ASE) to beremoved from a lasing medium, side faces of the medium are often groundso as to significantly reduce internal reflection of this light. In thecase of slab-type media, the two shorter side faces from which amplifiedstimulated light is not reflected are provided with this ground finish.Grinding generally results in a rough surface finish and a significantdepth of surface damage to the lasing crystal. Whilst the surfaceroughness may have an rms peak to peak amplitude of around 1 μm, surfacedamage may extend into the crystal by up to 20 μm (many times thewavelength of the lasing light) causing light, particularly pump light,exiting and entering the ground faces to be scattered by multiple bouncereflections. As the light gives up a given amount of energy per unitlength of its travel path, a relatively large amount of heat isgenerated at the ground faces (the pump light contributing the majorityof energy given up as heat). The present invention seeks in particularto reduce heat generation by light transmission at rough faces.

According to a second aspect of the present invention there is provideda laser comprising an elongate solid state lasing medium having aplurality of polished faces arranged to support lasing within the mediumat a laser wavelength and at least one non-reflecting face arranged toprovide for egress of ASE radiation from the lasing medium, wherein saidat least one non-reflecting face is provided with a surface finisharranged to minimise heat generation in the vicinity of thenon-reflecting face.

In one embodiment of the invention the or each non-reflecting face has arough surface finish, wherein the depth of surface damage produced isless than 5 μm but greater than 100 nm and more preferably greater than0.5 μm.

In an alternative embodiment of the present invention, the or eachnon-reflecting face is provided by a polished face of the lasing mediumand a layer of relatively low heat dissipating material covering atleast a portion of said polished face, the outer surface of the coveringlayer having a rough surface finish. The coating layer may compriseundoped lasing material or alternatively may be a dielectric whoseoptical properties are matched to those of the lasing material. Thecoating layer may be a thin slice of material bonded, e.g. by diffusionbonding, to the corresponding surface of the lasing medium.

In a first embodiment of the present invention, the lasing medium is aslab-type lasing medium having upper and lower polished faces forsupporting lasing and non-reflecting side faces to provide for egress ofASE radiation, the laser comprising an air gap surrounding the lasingmedium on the two non-reflecting side faces and on one of the reflectingfaces, and a relatively high thermal conductivity material contactingthe lasing medium over the other of the reflecting faces. Preferably,the laser comprises a source of pump light adjacent to the reflectingface of the slab which is not in contact with said high conductivitymaterial and means opposed to the non-reflecting side faces forreflecting pump light into the lasing medium whilst absorbing ASE light.Preferably, the source of pump light is surrounded on three sides byreflecting means for directing pump light from the source to the lasingmedium. More preferably, said reflecting means and the means forreflecting and absorbing opposed to the non-reflecting sides, compriseberyllia or alumina.

In an alternative embodiment of the invention, the laser may bedouble-pumped, i.e. with pump sources arranged on two sides of anelongate slab-type lasing medium. A liquid coolant may be containedbetween the pump sources and the medium.

For a better understanding of the present invention and in order to showhow the same may carried into effect, reference will now be made, by wayof example, to the accompanying drawings, in which:

FIG. 1 illustrates beam propagation along a slab-type lasing medium;

FIG. 2 shows a longitudinal cross-section through a solid state laseremploying a slab-type lasing medium;

FIG. 3 shows a transverse cross-section through the solid state laser ofFIG. 2; and

FIG. 4 shows a transverse cross-section through an alternative slab-typelasing medium suitable for use in the laser of FIGS. 2 and 3.

There will now be described with reference to FIGS. 2 and 3 a solidstate laser embodying the present invention. The laser comprises a slabof lasing material 5 which may for example be Nd:YAG. However, it willbe appreciated that any other suitable type of material may be used. Theslab is elongate having a rectangular cross-section, with the lower face5 a of the slab contacting a slab mount 6 which is of a high thermalconductivity material. A preferred material for the mount 6 is beryllia.The slab mount 6 is in turn mounted on an aluminium base plate 7. Due todifferent rates of thermal expansion of the slab mount and the baseplate materials, the two are kinematically mounted together in a knownmanner such that they are rigidly fixed to one another whilst stillallowing for relative displacement due to thermal expansion.

Energy to drive the lasing medium 5 is provided by a cerium doped quartzflash lamp 8 which is clamped between appropriately contoured sapphireblocks 9. The sapphire blocks 9 conduct heat away from the flash lamp 8whilst being substantially transparent to the light generated by theflash lamp. The sapphire blocks are bounded on their outwardly facingsurfaces by respective blocks of high conductivity material 10, againpreferably beryllia which combines high thermal conductivity, electricalisolation, and good diffuse light reflection. The beryllia blocks 10 inturn contact lamp heat sinks 11 which are thermally isolated from thebase plate 7.

The sides 7 a of the base plate 7 extend upwardly towards thenon-reflecting side faces 5 c, 5 d of the slab 5 and mounted to thesides of the base plate, opposed to the non-reflecting side faces of theslab, are reflector bars 12. Preferably, the reflector bars are coatedon their outwardly facing surfaces 12 a with a dielectric materialproviding a wavelength specific reflectivity. Whilst the reflector barspreferentially absorb ASE light which is transmitted by the coating,pump light is reflected back into the lasing medium by the reflectivecoating.

The upper and lower faces 5 a, 5 b of the slab are polished to anoptically smooth finish such that light is able to propagate in agenerally axial direction through the slab by a reflecting alternatelyoff the top and bottom faces (as illustrated in FIG. 1). In contrast,the side faces 5 c, 5 d of the slab (FIG. 3) have a finely ground finishsuch that light is generally not reflected by these faces and is able toexit the slab. By allowing ASE light to leak out through the side faces,the parasitic effect of this light is significantly reduced and energyis allowed to build up in an axial lasing mode. ASE light exiting theside faces 5 c, 5 d is substantially absorbed by the reflector bars 12whilst pump light exiting the side faces is reflected back into the slab5. Mirrors 13 are arranged at respective ends of the slab 5 and redirectlight exiting the slab end faces back into the slab.

It is generally the case that slab-type lasing media are provided with arough surface finish, often resulting from the cutting of the bulkmaterial into slabs. This finish typically results in an rms peak topeak amplitude of 1 μm or more. However, the roughening process producessurface damage, e.g. crystal fractures, down to a depth of 20 μm. Asdiscussed above, this depth of surface damage (sometimes termed the“surface damage zone”) results in an unacceptably high level ofscattering for light exiting and entering the slab which in turn resultsin significant amounts of heat being generated at the roughened faces.In order to reduce this problem, pre-roughened faces are first polishedto a relatively smooth finish (e.g. <100 nm) and are then reroughened orpatterned to provide a finish with a surface damage zone of depthcomparable to the wavelength of the lasing emission. Thus, for a lasingwavelength of 1 μm, the depth of the surface damage zone will be in theregion of 1 μm. This level of finish is sufficient to substantiallyprevent internal reflection of light at the roughened or patterned faces5 c, 5 d whilst also significantly reducing the level of scattering oflight exiting and entering the roughened faces.

As can be seen from FIG. 3, the slab 5 is surrounded on three sides byan air gap 14 and only makes contact with the slab mount 6 over thesurface of the lower face 5 a. The air gap is 3 mm or less in width andprovides thermal isolation for the slab 5 from the reflector bars 12 andthe flash lamp 8, whilst minimising the flow of convection currents. Thetemperature distribution within the slab 5 is therefore substantiallydetermined by the heat generation processes within the medium and by theslab mount 6. This arrangement achieves a reduction in temperaturegradients between the non-reflecting side faces of the slab as comparedto conventional arrangements where a multiplicity of factors affect thetemperature gradients within the slab.

As an alternative to reroughening the side faces 5 c, 5 d of the slab,it is possible to employ a slab of the type shown in FIG. 4. Followingpolishing of the side faces 5 c, 5 d, thin sections 15 of undoped lasingmaterial are diffusion bonded to the side faces of the slab 5. The outersurfaces 14 of the thin sections 15 are provided with a ground finish.As the optical properties of the doped and undoped lasing media aresubstantially the same, the boundary between the two media has littleeffect upon light crossing it. However, because the undoped lasingmedium possesses a relatively low light absorbtion coefficient, littleheat is generated at the outer faces 16 of the thin sections and even arelatively large level of scattering at the outer faces will result inonly relatively low levels of heat generation. Again, this facilitateseffective removal of ASE light whilst giving rise to little heatgeneration due to the passage of pump light.

As an alternative to using undoped lasing material to provide a finishfor the side faces of the slab, it is possible to use any suitablymatched dielectric material. This may be in the form of a thin sectionbonded to the side faces or may be provided by a thin film depositionprocess.

It will be appreciated by the skilled person that various modificationsmay be made to the above described embodiment without departing from thescope of the present invention. It will also be appreciated that theinvention is applicable in general to solid state lasers and is notlimited to lasers employing slab-type lasing media. For example, theinvention can be applied to reduce heat generation in cylindrical rodlasing media.

What is claimed is:
 1. A laser comprising an optically-pumped elongatesolid state lasing medium in the form of a slab having a rectangular orsquare cross-section, the slab having two opposed polished facesarranged to support lasing within the medium at a laser wavelength andtwo opposed non-reflecting faces arranged to provide for egress of ASEradiation from the lasing medium, wherein said non-reflecting faces areprovided with a surface finish achieved by polishing and thenre-roughening to produce a surface finish which scatters incident lightsuch that the depth of surface damage at the re-roughened faces is lessthan about five times the lasing wavelength and arranged to minimizeheat generation by ASE and pump light radiation in the vicinity of thenon-reflecting faces.
 2. A laser according to claim 1, wherein there-roughened surface finish has a depth of surface damage of less than 5μm but greater than 100 nm.
 3. A laser according to claim 2, wherein thedepth of surface damage is greater than 0.5 μm.
 4. A laser comprising anoptically-pumped elongate solid state lasing medium in the form of aslab having a rectangular or square cross-section, the slab having twoopposed polished faces arranged to support lasing within the medium at alaser wavelength and two opposed non-reflecting faces arranged toprovide for egress of ASE radiation from the lasing medium, wherein eachnon-reflecting face is provided by a polished face of the lasing mediumand a covering layer of relatively low heat dissipating materialcovering said polished face, the outer surface of the covering layerhaving a rough surface finish and the covering layer having thermal andoptical properties which are the same as or similar to those of thelasing medium.
 5. A laser according to claim 1 or claim 4, the lasercomprising an air gap surrounding the lasing medium on the twonon-reflecting side faces and on one of the reflecting faces, and arelatively high thermal conductivity material contacting the lasingmedium over the remaining one of the polished faces.
 6. A laser asclaimed in claim 5, comprising means opposed to the non-reflecting sidefaces and arranged for reflecting pump light back into the lasing mediumwhile absorbing ASE light.